The present invention relates to a circuit for controlling the power supply of an electrical load provided with a device for detecting a short-circuit of the load. More particularly, the invention relates to a circuit for controlling the power supply of an inductive load whose detecting device monitors the possible short-circuiting of the load in order that a corrective action may be triggered. Damage is thereby prevented, for example, damage to the circuit by a short-circuit of the load or defective functioning of a unit having this load.
In motor vehicle electronics, numerous actuators have such inductive loads. Such inductive loads are the winding of a fuel injector or that of an electromechanical valve for varying the pressure of a braking fluid in an antilocking device for the wheels of a vehicle. The inductive load may also be the coil of an inductive primary ignition circuit.
The circuits controlling such inductive loads in a motor vehicle environment are often fitted with diagnostic means enabling the detection, for example, of a disconnection or an accidental short-circuit of the inductive load which renders the latter inoperative, this detection being made for reasons of safety for example. These means are electronic circuits which monitor the voltage existing at the terminals of the load in order to detect, for example, the possible appearance of a short-circuit and to trigger alarms or appropriate corrective actions.
Considering the high interference environment of a motor vehicle, the voltage sampled at the terminals of the load must be filtered in order that it may be used, the filtered signal being compared with a reference voltage. When the filtered signal is below this reference voltage, the means used for comparison supplies a signal representative of a short-circuited state of the load.
FIG. 1 of the drawings shows a prior art circuit for controlling the power supply of an actuator having an inductive load. The circuit has a power transistor 1 controlled by its base in order to initiate the power supply of an inductive load 2 connected in series with the emitter-collector circuit of the power transistor 1, between the collector of the transistor 1 and ground. In the circuit shown, the power supply voltage +Vbat is supplied by the battery of a motor vehicle. As has been explained above, a filter 3, for example a low pass capacitive filter, takes the voltage from the terminal which is common to the inductive load 2 and the collector of the transistor 1 in order to filter out the interference from this signal before applying it to an input E of a control unit 4 which is designed to use the signal received from the filter 3. The unit 4 also controls the conduction of the power transistor 1 via a terminal S connected to the base of this transistor, through an actuator control sub-circuit 5.
When the control unit triggers this conduction, the capacitor C of the low pass filter 3 begins to charge with a certain time constant. In FIG. 3 of the accompanying drawing, there is shown at A the control signal Vs of the transistor 1 output at S by the control unit, and at B, the voltage V.sub.E at the terminals of the capacitor. Up to the point in time T where V.sub.E reaches a value Vref of a reference voltage generated by the control unit 4 in order to derive, from a comparison of the voltage at the terminals of the load 2 after filtering with this voltage Vref, the presence or absence of a short-circuit of this load, the comparison is not significant since the voltage at the terminals of the load is constantly lower than Vref, even in the absence of a short-circuit.
It is therefore necessary to provide a means for preventing the control unit from taking into account an short-circuit information u to the point in time T.
In order to do this it is possible to consider generating a delay of the output signal of the filter by analog or digital means. When the control unit is a microprocessor, the delay is obtained by means of a parallel loaded counter which is initialized by the microprocessor. Using this assumption, the voltage signal at the terminals of the load could also be filtered by a digital means.
The masking logic necessary for obtaining a digital delay has the disadvantage of being expensive. In fact, the delay to be generated must in practice be able to reach values in the order of 200 .mu.s which, for a resolution of 2 .mu.s, involves the use of a parallel loaded 7-bit counter in order to be programmable. A requirement which additionally requires a 7-bit register and the associated logic. This assembly requires the integration of about 200 MOS transistors per counter resulting in an expensive production.
Such a solution does not however provide a reduction in the length of the time interval during which a short-circuit of the load cannot be detected. As described above, the generated delay must in practice be in the order of 200 .mu.s, whereas experience shows that a short-circuit of the load for more than 100 .mu.s which is not taken into account can affect the safety of the control circuit of the power supply of this load.